Precambrian tectonics lecture - Greenstone
General characteristics of greenstone belts:
- North American, South African, Australian
and Indian cratons all display greenstone belts.
- they often occur in arrays of parallel greenstone
belts, separated by tonalite-metamorphic realms.
- they are mostly Archean in age, but with
some Proterozoic versions.
- average dimensions - 20-100 km wide, and
up to 100s of kms long.
- structurally they are primarily synclinoriums
with steep foliations.
- contacts are usually synkinematic plutons
with a dynamic intrusive style, but in S. Africa see that part
of the greenstone sequence was deposited on basement rocks.
- polyphase deformation common. Some argue that with more detailed mapping
can work out internal thrust stacks, suggesting significant horizontal
- filled with volcanics and sediments in manner
suggesting they represent a long term basin, one deep enough
to receive turbidites.
(hence the color and the name) to amphibolite metamorphic
facies, with a decrease of grade into the interior of the
- while an individual greenstone belt may not
be unique to the Precambrian, the formation of parallel arrays
of them is!
Internal stratigraphy of greenstone belts:
- 10-20 km thick sections described (real or due to tectonic repetition?).
- 2 common units:
- upper sedimentary unit: usually 15-30% of total section. Made up of graywackes,
cherts, argillites, BIFs, with some arkose and cg at very top.
- lower volcanic unit:
see an overall evolution in the volcanic pile with a lower
bimodal suite and an upper calc-alkaline suite. The
lower suite often has pillows and can have komatiites,
with minor rhyolites. Higher up andesites and dacites exist.
Evidence of cyclicity.
Some examples and images of components:
Barberton belt in S. Africa is a classic and well studied example of a greenstone
belt. Deformation and metamorphism or less intense here so the
earlier basin history is clearer.
Eli greenstone belt of Minnesota: see below
images and text.
This is an image of pillow basalts in
the Eli greenstone belt in northern Minnesota. One can see the
more pointed or tapered bottoms and the more bulbous tops of the
pillows, indicating a younging to the left (South). The finer
grained, once glassy rims are now a darker color, while the interior
is weathered white. Glacial striations can also be seen on the
outcrop surface. Camera lense in center for scale.
This is an image of a hypabyssal and phenocrystic
dacitic intrusion in part of the Eli greenstone belt. To the left
are the fine-grained metasediments the dacite intruded into. Note
the small xenoliths in the dacite. Intermediate composition volcanics
(dacites and andesites) are a common rock type in the area. Metamorphism
is relatively mild and original igneous textures are preserved.
This is a glaciated flatrock exposure from
the Eli greenstone belt literally just down the road from the
above outcrop. Exposed are graded volcanic wackes layers intercalated
with muds, suggesting these are turbidites in a basin next to
a volcanic chain. They are upright here and elsewhere show evidence
of both soft-sediment and tectonic deformation. One can also see
a nice branching fault termination here.
Models for greenstone belts
Are they collapsed impact basins (doubtful)?
Are they accreted arcs?
- volc. signature is estimated at 57% tholeiitic,
38% calc-alkaline, and 5% alkalic on average.
- perhaps a primitive arc? Modern arcs and
greenstone belts do have similar REE pattern.
- doesn't explain initiation and the long history
as a basin.
Are they collapsed rear-arc basins?
- Mesozoic Rocas Verdes may represent younger analog in southern tip of S.
- Carolina terrane in the southern Appalachians
- would explain the basin and tholeiite aspects.
- would also explain the collapse and subsequent
tonalite/granodiorite intrusions as collapse of basin and reestablishment
of arc magmatism.
- why not more common in the Paleozoic? Why
in parallel arrays, common age?
Are they collapsed continental rifts?
- idea would be they are rifts but on thinner,
hotter continental crust and in a permobile regime so that collapse
and subsequent intrusion would cause the pervasive deformation
that is seen. The craton would thicken, cool and rigidify during
and after this.
- would explain the basin history, but not
the calc-alkaline volcanic component.
Are they polygenetic?
Course materials for Plate Tectonics, GEOL
3700, University of Nebraska at Omaha. Instructor: H. D. Maher
Jr., copyright. This material may be used for non-profit educational
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